1. Field of the Invention
The present invention in general relates to a manufacturing method for improving optical coupling characteristic. More specifically, the present invention relates to a manufacturing method of automatically coupling between a fiber and an optical waveguide, which can promote the coupling efficiency between the fiber and the optical waveguide by controlling accurately the width and the depth of grooves for supporting the fiber. Alternatively, such a process also reveals high reproducibility.
2. Description of Related Art
Optical waveguide chips, which are planar optical circuits consisting of optical waveguides, are fabricated on a silicon chip and mostly utilized in optical fiber communication and other related optical systems. FIG. 1 shows the construction of a conventional general optical waveguide chip. Numeral 10 is a silicon substrate and a waveguide 16 is sandwiched between a waveguide buffer layer 12 and a waveguide cladding 14. Generally, the width and height of the waveguide are about 8 .mu.m; the thickness of the waveguide buffer layer, about 20 .mu.m; and the thickness of the waveguide cladding, about 20.about.30 .mu.m. Both end faces of waveguide 16 that can perform a specific function must be coupled to a fiber in order to connect to external optical circuits or devices. Common waveguides are usually fabricated from high-silica glass because such a waveguide displays low loss in lightwave transmission and in coupling due to the mode field of the high-silica waveguide being similar to that of the commercial single-mode fiber. On the other hand, the interface between the high-silica glass and the silicon substrate displays excellent chemical bonding capability.
Therefore, how to accurately couple the end faces of the waveguide in a waveguide chip to a fiber linking with external optical devices and reduce the coupling loss is a very critical issue. The main approach adopted now is to arrange the fiber in an aligning groove prepared by etching technique so that the core of the fiber will finely tune to the geometric center of the corresponding waveguide. The accuracy and precision of aligning the fiber with the waveguide will affect the optical coupling loss resulting from misalignment between both. FIG. 2 is a relationship diagram of the alignment error and the coupling loss while coupling between a single-mode fiber and an optical waveguide, where the core radius of the single-mode fiber is 9 .mu.m and the difference of the refraction index 0.25%, the width and height of the optical waveguide are all 8 .mu.m, the difference of the refraction index 0.3%. The coupling loss of 0.3 dB can be considered reasonable while the alignment error is about .+-.1 .mu.m. However, it is almost impossible to achieve such a tolerance requirement by the above-mentioned conventional fabrication process, especially when making aligning grooves by reactive ionic etching (RIE) that can not precisely control the depth of the grooves.
The conventional aligning technique can fundamentally be divided into two approaches. The first approach is called static alignment, which is characterized in that the requirement of alignment for the fiber and the corresponding waveguide is achieved when the alignment groove finishes. This type of alignment approach, like the above-mentioned aligning technique, is quite straight and known as a best method of optical coupling currently. U.S. Pat. No. 5,297,228 discloses a static alignment method of optical coupling. In the '228 patent, an end face of a waveguide chip is abutted against an end face of a fiber aligning jig on which optical fiber are arranged. At least one marker is formed in each of the waveguide chip and the fiber aligning jig, and also at least one pin guide groove is formed in each of the waveguide chip and the fiber aligning jig, using the marker as a reference mark. The planar optical waveguide and the optical fiber are aligned with each other by means of a common guide pin laid along the corresponding pin guide grooves. The above-mentioned grooves are usually formed by a dicing saw. However, the performance of the dicing saw may be seriously degraded due to long-term operations and then such a circumstance will affect accuracy of width and depth of the cutted grooves and increase the coupling loss.
The second approach called active alignment is apparently different from static alignment. Aligning in active alignment is made by measuring the optical characteristics between the fiber and the waveguide and adjusting their relative positions, for example, disclosed in U.S. Pat. No. 5,175,781. Using a laser ablation system, predetermined positions for aligning grooves can be formed in the wafer. Fine dynamic adjustment of the predetermined positions can be made by the above-mentioned measuring and adjusting process. The chief disadvantage of this aligning method is that it is difficult to control the vertical depth of the aligning groove. On the other hand, in practice, it is hard to be implemented so as to increase the manufacture cost and reduce the efficiency of production.
Another approach that utilizes dry etching to cut off the desired aligning grooves had been disclosed in Journal of Lightwave Technology LT-5, No. 12, P1716(1987). However, with such a scheme, it is difficult to maintain the required accuracy in depth, especially using reactive-ion etching (RIE).